The present invention relates to a golf ball mold and a golf ball manufacturing method which may be suitably used for molding golf balls composed of a core encased by a cover of one or more layer, particularly for forming an outermost cover layer having a plurality of dimples formed on the surface thereof.
In recent years, there has been a strong desire for golf balls which possess various performance attributes, including not only distance, but also controllability, durability and feel on impact. Satisfying all of these attributes with only one type of material is generally difficult. Hence, the customary practice is to provide the ball with a structure in which a solid core formed of rubber, resin or the like, or a wound core, is encased by a cover of one or more layer, each layer having a particular function. In other words, efforts have been made to achieve a performance which addresses the needs of the player, such as a desire for distance or controllability, by adjusting the number and thickness of the above layers, and also adjusting the formulations, etc. of the materials making up the respective layers.
An injection-molding method is typically used to form the outermost layer of a golf ball having such a structure. Specifically, use is made of a method in which a core or a sphere (referred to below as a “center sphere”) composed of such a core encased by one or more intermediate layer (a layer other than the outermost cover layer) is placed in the cavity of a given mold, and a cover-forming material is injected between the center sphere and the inner wall of the cavity. In this case, when the outermost layer is formed, numerous dimples are formed at the same time by numerous dimple-forming protrusions provided on the inner wall of the cavity.
Up until now, molds having the structure shown in FIG. 6 have often been used when producing golf balls by the above process. FIG. 6, a cross-sectional view showing an example of a golf ball mold according to the prior art, depicts the state prior to injection of the cover-forming material into the cavity.
In FIG. 6, a conventional mold 10 includes a mold body 20 having an upper mold half 20a and a lower mold half 20b which split at a parting surface that defines a parting line PL at a golf ball equator and removably mate to form a cavity 3 having an inner wall with numerous dimple-forming protrusions thereon, and includes support pins 40, each having on an end face thereof a single dimple-forming projection, which are extendable into and retractable from the cavity 3. The support pins 40 extend into the cavity 3 to support a center sphere 31 and, in the retracted state, the end faces thereof define a portion of the inner wall of the cavity 3. Moreover, although not fully shown here, the support pins 40 each have a circular cross-section, and a total of six pins—three in the upper mold half 20a and three in the lower mold half 20b—are provided so as to be spaced at given intervals at positions having 120 degree rotational symmetry about the pole Q as the center.
In the above mold 10, runners 50 and resin injection ports 60 having openings of given surface areas are formed along the parting surface of the mold body 20 in such a way as to inject, between the inner wall of the cavity 3 and the center sphere 31, a known cover-forming material from a known injection molding machine (not shown). Next, together with injection of the cover-forming material, the support pins 40 that were extended into the cavity are retracted, after which cooling is carried out, thereby completing formation of the cover. The dimples at the positions of the support pins 40 are formed at this time by the dimple-forming protrusions that were formed on the end faces of the support pins.
However, when the center sphere 31 is placed in the above mold 10 and the upper and lower mold halves are closed, the support pins 40 may be subjected to excessive forces, causing them to deflect or shift, as a result of which irregular flash sometimes forms on the surface of the molded ball. When irregular flash arises on the ball's surface, uniform trimming of the entire ball is difficult to carry out, which may lead to dimple irregularity and ultimately have an adverse effect on flight symmetry.
Moreover, because the end faces of the support pins 40 are generally obtained by machining the end face of a cross-sectionally circular member at an angle so as to impart a shape which defines a portion of the inner wall of the cavity 3 when the support pin is in a retracted state, the dimple-forming protrusion formed there often has an elliptical shape as seen from above. Such elliptically shaped dimples may sometimes adversely affect the ball's appearance, and are a major factor in lowering the degree of freedom in dimple design and mold design.
To address this problem, JP-A 08-323772 and U.S. Published Patent Application No. 2009/0297653 disclose golf ball molds wherein a center sphere placed in the mold is not supported using a plurality of thin support pins like those shown in FIG. 6; instead, a large-diameter support pin having on an end face thereof a plurality of dimple-forming protrusions is provided. However, in these molds, although the stability when supporting a center sphere improves, because the diameter of the support pin is too large, gases tend to collect near a pole of the cavity (at the center of the end face on the support pin) during injection molding, which may give rise to appearance defects.
In order to resolve the above problem of appearance defects, JP-A 2002-542067 (and the corresponding U.S. Pat. No. 6,129,881) and U.S. Pat. No. 7,341,687 disclose molds in which the venting of gases near the poles of the cavity has been improved by providing a gas-venting pin in the large-diameter support pin. However, the venting pin and the support pin must be separately fabricated, resulting in excessive costs.
In this way, various modifications have been made to golf ball molds so as to improve the golf ball moldability, but a fundamental solution has yet to be found for the above problems. Accordingly, for the sake as well of further improving the golf ball moldability and the degree of freedom in dimple design and mold design, there exists a desire for a novel approach which is capable of resolving the above-described problems.